Transition metal complexes of disulfides and sulfur-containing organic ligands are being prepared, characterized by spectroscopic and analytical methods, and studied crystallographically. Ligands and metal ions are selected for their potential relation to the structure and composition of the active sites of some metalloproteins. Particular effort is being directed at copper in its oxidases such as ceruloplasmin and the laccases, and at the relation between the Cu(II) - Cu(I) couple of the disulfide group. A preference of Cu(I) for N3S coordination over N4 or N2S2 coordination has been noted, as has the ability of Cu(II) to catalyze the scission of the SS bond, to give Cu(I) and a sulfinic acid. Sulfinic acid complexes of Co(III) and Cu(II) have been isolated and studied crystallographically.